Imaging consists in illuminating an object using a source of radiation, often of high energy, and of forming an image on the basis of the radiation that is back-scattered (especially in the case of objects opaque to the radiation) or transmitted. The radiation considered here is for example radiation that penetrates the material, typically X-rays, or even “gamma” rays, but may also be what is referred to as THz radiation, that is to say radiation of which the frequency, of the order of the terahertz, situates it between the infrared and the microwave domain. These concepts of illumination or imaging do not in any way imply that the radiation is in the visible domain (generally it is the opposite case); the term irradiation is sometimes used, in the case of certain penetrating radiation.
There are two broad families of imaging devices according to the illumination configuration used:                Imaging devices with “imager”, in which the source fully and uniformly illuminates the object; the detection of the back-scattered radiation is carried out by a pixelated imager of large format associated with an image forming system of greater or lesser complexity,        “Scanning” devices in which the source illuminates part (in practice a small zone) of the object at a given time and performs scanning in order to cover the whole of that object during a scanning cycle. Collimation is generally placed in front of the source in order to reduce the angular amplitude of the illumination beam at each instant and thus the illuminated zone. The back-scattered radiation (and/or the transmitted radiation) is collected by one or more one-dimensional detectors synchronized with the scanning by the source.        
There are a wide variety of imaging devices with imager.
There are thus imaging devices with a virtual imager (reconstitution by scanning of the field of view of a detector, in practice one-dimensional, which is strongly collimated, and of which movement is provided using Cartesian or polar coordinates), and imaging devices with a matrix imager (this matrix imager is formed from a matrix of strongly collimated one-dimensional detectors). Developed before the appearance of large detection matrices, they are still used in in vitro scintigraphy and for the examination of small laboratory animals. These devices lack spatial resolution given the size of the unit detector and the collimation used to form the image.
Imaging devices with imager were then developed thanks to the appearance of pixelated imagers; a particularly simple version is the pinhole imager, that is to say it employs a hole of small size formed in a screen interposed between the illuminated object and the detector. The spatial resolution increases when the size of the hole reduces such that the main drawback of this type of imager is that an increase in spatial resolution implies a reduction in the quantity of radiation transmitted by the pinhole; this leads either to very long exposure times, or to images of poor signal-to-noise ratio.
To improve the signal-to-noise ratio, it is possible to use the technique of penumbra imaging, in which the pinhole is replaced by an aperture of greater size (in the shape of a disk, or in a ring, for example). The quantity of radiation received by the detector is higher but the image is blurred and must be deconvoluted by mathematical processing using the knowledge of the aperture shape and of the detection geometry to determine the deconvolution kernel.
A second way to get around the limitations of the pinhole principle is the coded aperture technique, consisting of having multiple pinholes within the same screen, also called a mask; the quantity of radiation reaching the detector increases with the number of apertures. A reconstruction operation must then be carried out to reconstitute the image of the object. The distribution of the pinholes within a given mask must satisfy a certain number of rules to facilitate that reconstruction of the image and minimize the contribution of the noise.
The principle of the method has been described by: E. Fenimore and T. M. Cannon “Coded aperture imaging with uniformly redundant arrays”, Applied Optics, Vol. 17, No. 3, p. 337-347, Jan. 2, 1978; several patents disclose devices implementing this coded aperture technique:                U.S. Pat. No. 4,389,633 (Fenimore et al) published in 1983,        PCT Pub. No. WO 97/45755 published in 1997,        PCT Pub. No. WO 02/13517 published in 2002,        PCT Pub. No. WO 02/056055 published in 2002,        U.S. Pat. Pub. No. 2004/218714 published in 2004 and        PCT Pub. No. WO 2007/091038 published in 2007,        
The main limitations of the coded aperture devices are their high cost since the large-size masks are costly to produce while the associated imagers must have dimensions at least equal to those of the mask. Their spatial resolution is not adjustable since it is linked, in particular, to the size of the holes in the mask and to the detection geometry.
An original device, described in PCT Pub. No. WO 2007/015784, consists of using X-ray focusing optics of very particular form enabling the radiation scattered by the object to be focused onto an imager of small size; however, the X-focusing optics do not operate for all radiation energies and its efficiency is rather low.
The other category of imagers, referred to as “scanning” imagers, also comprises several variants.
Thus, the simplest version consists of illuminating the object with a radiation beam (typically X-rays) of very small diameter (the term “pencil beam” is sometimes used) and of scanning the entire object by virtue of a mechanical system; the dimension of the radiation beam and the scanning step size determine the spatial resolution; such scanning is typically carried out by means of an annular ring movable around its center, pierced with collimation passages, the source of radiation being fixed, situated at the center of this annular wall; this ring may be of greater or lesser thickness depending on whether it is a wall pierced with holes or a disc holed in its center, in which radial channels are formed; this ring is sometimes called a chopper wheel. The rotation of this ring causes scanning of the beam perpendicularly to the rotational axis of the ring; the other movement ensuring the scanning of the object is conventionally provided by a movement of the object parallel to the rotational axis of the ring. A detector is placed beside the source; it records a series of signals which are then provided in matrix form to constitute the back-scattered image of the object.
Such a configuration, and examples of application are in particular described in U.S. Pat. No. 5,764,683, and PCT pub. Nos. WO 00/33060, WO 01/94984, WO 2004/043740 or WO 2006/102274.
According to a variant, the pencil beam results from the interception of a fan beam which is intercepted by a wheel of which the axis is parallel to the plane of the beam and which comprises radial slits; such a configuration and examples of application are in particular described in U.S. patents Re 28 544, U.S. Pat. No. 4,031,545, U.S. Pat. No. 4,799,247, U.S. Pat. No. 5,179,581, U.S. Pat. No. 5,181,234 or U.S. Pat. No. 6,094,472.
Instead of providing scanning by movement of a pencil beam in two transverse directions, it is also possible to generate a fan beam which is passed through by the object to observe, as is for example described in PCT pub. No. WO 98/20366. It is to be noted that the reconstruction of an image based on such scanning may involve coding similar to that indicated above in connection with the imaging devices with imager.
Variants implement both types of aforementioned scanning devices (by a pencil beam and by a fan beam), as is proposed by the PCT pub. Nos. WO 99/39189 and WO 2008/021807.
The devices described above serve in practice for the inspection of parcels, baggage, even freight trucks or people.
The drawback of this scanning configuration is in practice to use only a very small part of the radiation emitted by the source since the spatial resolution improves the finer the illumination beam; the source must therefore be of high intensity and the detection surface of large size if acquisition times of reasonable length are desired.
An increase in the quantity of radiation used may be obtained by increasing the number of beams; this may be provided by the coding of a localized source with absorbent masks comprising holes; it is then necessary to reconstruct the image by matrix inversion techniques. A simple version (with a plurality of pencil beams simultaneously formed by means of a chopper wheel, combined with movement of the object) is described in PCT pub. No. WO 01/94894. To obtain a good result, in particular when the scanning is provided in two transverse directions, the collimation holes forming the beams must satisfy disposition rules; the best-known principle consists of projecting masks and their opposites constituting the elements of a Hadamard matrix; this imaging method requires little computation power (it is a simple matrix multiplication) but its practical implementation is very complex. Typically, for an image of size N×N, it is necessary to project 2·N2 masks. The resolution of the final image depends on the size of the smallest projected pattern; the more it is sought to have details in that image, the higher the number of masks to project has to be; mechanically these masks must be produced and positioned with high precision; such a system is extremely complex and costly (see PCT pub. No. WO 2008/127385 published in 2008).
Another approach consists of performing coding in relation to an area source, with a pinhole to project the pattern onto the object. In the same way as previously, it is necessary to have a succession of different patterns to acquire the data then reconstruct the back-scattered image (cf. U.S. Pat. No. 6,950,495 published in 2005).
These coding devices are extremely complex since a source of large area must be produced with zones (pixel) that can be modulated individually for emission; furthermore, the emitted intensity must be sufficiently high to enable the projection of the patterns via a pinhole, which considerably attenuates the signal; as previously, the resolution depends on the size of the smallest projected pattern.
Thus, the known solutions implement scanning illuminating devices which are generally voluminous and complex from the mechanical point of view, particularly as the scanning in two perpendicular directions in practice requires movement of the object relative to the radiation source. Furthermore, these scanning illuminating devices in practice enable a high spatial resolution to be obtained only provided a beam of small dimensions only is applied to the object.